CHAPTER VIII. INSTINCT.

1. INSTINCTS COMPARABLE WITH HABITS, BUT DIFFERENT IN THEIR ORIGIN. (continued)

If we suppose any habitual action to become inherited--and it can be shown
that this does sometimes happen--then the resemblance between what
originally was a habit and an instinct becomes so close as not to be
distinguished. If Mozart, instead of playing the pianoforte at three years
old with wonderfully little practice, had played a tune with no practice at
all, be might truly be said to have done so instinctively. But it would be
a serious error to suppose that the greater number of instincts have been
acquired by habit in one generation, and then transmitted by inheritance to
succeeding generations. It can be clearly shown that the most wonderful
instincts with which we are acquainted, namely, those of the hive-bee and
of many ants, could not possibly have been acquired by habit.

It will be universally admitted that instincts are as important as
corporeal structures for the welfare of each species, under its present
conditions of life. Under changed conditions of life, it is at least
possible that slight modifications of instinct might be profitable to a
species; and if it can be shown that instincts do vary ever so little, then
I can see no difficulty in natural selection preserving and continually
accumulating variations of instinct to any extent that was profitable. It
is thus, as I believe, that all the most complex and wonderful instincts
have originated. As modifications of corporeal structure arise from, and
are increased by, use or habit, and are diminished or lost by disuse, so I
do not doubt it has been with instincts. But I believe that the effects of
habit are in many cases of subordinate importance to the effects of the
natural selection of what may be called spontaneous variations of
instincts;--that is of variations produced by the same unknown causes which
produce slight deviations of bodily structure.

No complex instinct can possibly be produced through natural selection,
except by the slow and gradual accumulation of numerous, slight, yet
profitable, variations. Hence, as in the case of corporeal structures, we
ought to find in nature, not the actual transitional gradations by which
each complex instinct has been acquired--for these could be found only in
the lineal ancestors of each species--but we ought to find in the
collateral lines of descent some evidence of such gradations; or we ought
at least to be able to show that gradations of some kind are possible; and
this we certainly can do. I have been surprised to find, making allowance
for the instincts of animals having been but little observed, except in
Europe and North America, and for no instinct being known among extinct
species, how very generally gradations, leading to the most complex
instincts, can be discovered. Changes of instinct may sometimes be
facilitated by the same species having different instincts at different
periods of life, or at different seasons of the year, or when placed under
different circumstances, etc.; in which case either the one or the other
instinct might be preserved by natural selection. And such instances of
diversity of instinct in the same species can be shown to occur in nature.